Langmuir-Blodgett Film, LB-film

(Study LB films with KSV LB Troughs)

Definition:

An insoluble spread monolayer of atoms or molecules floating at the gas/liquid interface and subsequently transferred onto a solid substrate.

Summary/Description:

Lipids, polymers or other water insoluble atoms or molecules can form ultrathin and organized monolayers at the air/water interface i.e. Langmuir films These films can be deposited on solid substrates to form highly organized regular multilayer stacks called Langmuir-Blodgett films. The LB films are prepared by successively dipping a solid substrate up and down through the monolayer at a constant molecular density or surface pressure. In this way multilayer structures of hundreds of a few nanometer thick monolayers can be produced. Irving Langmuir and Katherine Blodgett founded the science of LB films early in the 20th century.

The LB deposition is traditionally carried out in the "solid" phase. The surface pressure is then high enough to ensure sufficient cohesion in the monolayer, e.g. the attraction between the molecules in the monolayer is high enough so that the monolayer does not fall apart during transfer to the solid substrate. This also ensures the build up of homogeneous multilayers. The surface pressure value that gives the best results depends on the nature of the monolayer and is usually established empirically. However, amphiphiles can seldom be successfully deposited at surface pressures lower than 10 mN/m, and at surface pressures above 40 mN/m collapse and film rigidity often pose problems. When the solid substrate is hydrophilic (glass, SiO₂ etc.) the first layer is deposited by raising the solid substrate from the subphase through the monolayer, whereas if the solid substrate is hydrophobic (HOPG, silanized SiO₂ etc.) the first layer is deposited by lowering the substrate into the subphase through the monolayer.

There are several parameters that affect on what type of LB film is produced. These are the nature of the spread film, the subphase composition and temperature, the surface pressure during the deposition and the deposition speed, the type and nature of the solid substrate and the time the solid substrate is stored in air or in the subphase between the deposition cycles. Different kind of LB multilayers can be produced and/or obtained by successive deposition of monolayers on the same substrate. The most common one is the Y-type multilayer, which is produced when the monolayer deposits to the solid substrate in both up and down directions. When the monolayer deposits only in the up or down direction the multilayer structure is called either Z-type or X-type. Intermediate structures are sometimes observed for some LB multilayers and they are often referred to be XY-type multilayers.